Any wastewater that suffers from the afflictions above can profit from covers, but there are many sources of wastewater that almost certainly should be enclosed for safety's sake, operational efficiency, and cost recovery. Examples are municipal and industrial wastewater treatment plants, metal and petrochemical plants, leaching ponds, and airports. Notice that this list includes both biological and non-biological waste:
- Mining
- Rendering
- Food & Beverage Processing
- Agricultural Production
- Pulp & Paper
- Chemical Processing
- Petroleum & Gas
- Leather Manufacturing
- Military Production
- Landfills
- Biogas Generation
Challenges
Covering wastewater storage sites presents its own set of challenges. Since there are different types of covering structures, it's worth taking a good look at your site's needs and comparing each option's relative strengths and weaknesses.
Wind
Wind can spring up virtually any time and anywhere. Gentle breezes can give way to a stiff wind and, eventually, hurricanes or tornadoes if you're having an exceptionally challenging day. Some wastewater managers shy away from flexible wastewater covers because they imagine them to be, essentially, very expensive (and sometimes dangerous) kites. Flexible covers that end up draped over the windbreak down the road after a brisk day in March aren't doing their job.
Properly designed and engineered flexible covers are anchored to the banks or sides of the pond, reinforced, and securely fastened so that they are not subject to lifting or kiting. Floating geomembranes, in particular, are well-suited to resist the damaging effects of wind. Since they float directly on the water's surface, there is no space for wind to slip under the cover and lift it. This physical contact with the surface also impedes wave action. Without this protection, wavelets and water movement along the shoreline of in-ground ponds can lead to bank erosion, increased turbidity, and eventually bank collapse.
There are many benefits to covering wastewater treatment ponds, so it's essential to consider all your options, including flexible and floating covers when deciding whether this solution is practical for you.
Access
While pond covers represent a barrier to wildlife, some plant engineers may hesitate to cover wastewater impoundments out of concern that regular access will be awkward and time-consuming. As treatment progresses, it's necessary to have a means of entry to wastewater tanks to take measurements, perform inspections, adjust and clean equipment, remove problematic sludge, and add chemicals. Anything that substantially reduces efficiency in these activities is unlikely to be approved. Fortunately, well-designed tank and pond covers offer easy and convenient options for access. Rigid covers will generally include hatches or easy-access portals. Many flexible cover designs allow for complete removal and may feature limited access and automatic mechanisms to partially or fully retract the cover. Some newer, flexible covers consist of disconnected free-floating balls or plates that fit loosely together on the water's surface and which can be pushed aside or scooped out as needed.
Durability
Covers for wastewater impoundments need to be exceptionally durable - failure due to worn or damaged materials can be expensive and time-consuming to repair or replace. All cover types (rigid, flexible, distributed) should withstand wind forces and exposure to chemicals that make up the wastewater. Flexible materials must also be able to resist stretching and flexing, and along with ball or plate covers, must be reinforced to withstand wear from friction, temperature extremes, and UV exposure. If you're considering a flexible geomembrane cover, look for a material with a reinforcing scrim layer to offer extra support and wear resistance.
UV Exposure
Wastewater covers are exposed to sunlight 100% of the time. Rigid covers like metal and concrete are naturally suited to this kind of exposure, but flexible geomembrane covers made from plastics and polymers, are vulnerable in varying degrees to ultraviolet exposure. PVC is particularly subject to UV damage and quickly becomes brittle. Polyethylenes are better suited to long-term exposure, but even they have varying levels of resistance. When you're considering flexible wastewater covers, look for a geomembrane that has protection built in, with UV-protective coatings or a UV-resistant layer.
Installation
Installation varies according to the type of cover, the materials, and many other factors. Metal and concrete covers will require heavy construction equipment, cranes, welding equipment, and more. Disconnected floating covers like balls and hex plates are often delivered in bags and poured in, but there must be enough to ensure complete coverage. Flexible geomembrane covers are held in place using floats and tensioners and may include mechanisms for gas capture and other specific functions. Both disconnected and solid geomembrane floating covers can be installed quickly without the need for heavy equipment or interrupting facility operations.
Aeration
Mechanical aerators pump air below the surface of wastewater and distribute dissolved oxygen throughout the body of water. Dissolved oxygen supports the work of aerobic bacteria, which quickly feed on the remaining organic solids and produce carbon dioxide and odor-free biomass, which makes excellent fertilizer. This process, known as polishing, is one of the final steps before the water is released from the treatment facility.
Covered ponds and containers restrict or eliminate interaction between air and water, reducing oxygen interchange. While this is beneficial during the anaerobic digestion step, it does present challenges for aerobic processes. Steel and other rigid covers will need pipes, pumps, and vents to introduce oxygen, mix the water, and remove carbon dioxide. Operators can scoop out and store disconnected floating ball covers to approximate the area of uncovered surface needed for oxygen exchange. Or they can partially retract floating geomembrane covers to set up a mechanical aeration system or uncover the appropriate surface area.
Chemical Resistance
Wastewater comes from many sources, and each produces a unique set of challenges. Mining operations generate water with acids and heavy metals that can react with steel and plastic containers, many kinds of polymers, and even floating balls and plates. Biological waste from hospitals or sewage from livestock operations is equally unsuited to certain geomembranes.
Wastewater covers that make direct contact with the water, as in the case of floating covers, must be impervious to chemical and biological degradation from the wastewater itself. Even covers that sit above the water level, as in rigid structures or gas capture setups, must be resistant to exposure from water movement and splashing, equipment cleaning, and other maintenance activities. Since failure comes at such a high cost, it's essential to choose materials that can take on the challenges of containing corrosive or damaging liquids. You don't want to learn that a specific polymer isn't a good fit for storing acidic mining waste by discovering that a severe leak in your lagoon or tank has made its way to a nearby creek.
